I think a bike fit takes a lot of the guesswork out of your hands, it also gives a placebo esque feeling of satisfaction that everything is ok.
Thing is, bike setup is SO important. If you get it wrong, you get knee ache, back ache or any number of aches, and it could be a matter of cm's that makes the difference.
Since having the cleats Ive had pretty bad knee pain in the right leg that goes on for about 3-4 days pretty much stopping me from riding consecutively. I took a risk before the London To Brighton on Sunday by following the heel on the pedal trick and consequently raised my saddle about 2 cms (pretty significant). I finished the ride and the next day my knee was spot on, no pain at all.
I DID however get a little back pain going up the big one, which I haven't had on big climbs before, so my saddle perhaps needs dropping, maybe even a matter of mm's.
This extreme fine tuning can be done on your own, but I believe a bike fit, done right, would negate most of this trial and error. I'm not sure how most of them work, but I know the Specialized Concept Store ones end up setting your bike up and also giving you a document with exact frame / seat / stem etc measurements to the mm. Whenever I get my bike serviced I would plan on taking this sheet and saying, when done this is how I want it!
Guess what I want to say is if you want to get it right straight away, and not go thru trial and pain, they are probably worth every penny, especially when your balancing cycling with a busy schedule.